Repairing Mopar Rust - Metal Magic

Of the many attributes of steel, the major component used in construction of the body and structural members of all but some of the most recent Mopars, one downside of this product is certain: Steel will rust. And if the Mopar you’re driving or restoring is like some of the ones we’ve seen lately, chances are it suffers from a rust hole or two in one of the body or floor panels. Even with the best paint job, a tiny imperfection due to a rock chip or scratch can allow water to migrate to the metal, causing rust, which if left untreated can lead to panel rust-through.

2/13How badly is your Mopar rusted? This month we’ll show you how to repair that little, or big rust hole in your car the right way.

While some car manufacturers got a bad reputation for their cars rusting more than others, in reality it depended more on where in the world the car was located, and whether or not it was garaged, cleaned regularly, and driven on salty roads. In our state of Florida we see some extremely rusty cars from salty coastal locations as well as some surprisingly rust-free Mopars from the inland parts of the state. But as these cars continue to age, even a “rust-free” Mopar is likely to have an area or two that needs attention, and we don’t mean filling the rust hole with a gallon of body filler. We mean repairing it properly, with metal.

To properly repair rust, the rusted area must either be cut away completely and replaced, or ground down and treated, depending on the severity and location of the rust. Minor scaling and pitting on body and floor panels can generally be removed with a grinding stone on a power grinder, while major areas of rust-through and rusted frame or structural parts require completely removing the affected area and welding in new steel.

3/13<strong>1.</strong> Rust can be ugly, but with the proper tools and equipment it can also be repaired. If your Mopar is rusty, we suggest you get rid of it (the rust, not the Mopar) sooner and not later, as rust can spread to consume entire panels or even framerails.

Either way, the surrounding area must be chemically treated with a rust neutralizer containing phosphoric acid to ensure any remaining surface rust doesn’t reappear through the new paint, causing unsightly bubbles. Depending on the size and number of the rusted areas needing to be repaired, at some point the decision needs to be made about whether it’s less work and expense to simply replace the entire body panel. Of course this method only works if the panel is available for your vehicle, and fortunately companies like Auto Metal Direct offer new panels for most popular Mopars.

Once you’ve decided to repair a rusted area, beware of what you find underneath. Areas like the lower rear quarter panels are welded to an inner quarter panel, which might be rusted as well. If the rusted area you’re repairing is small enough, it’s fairly easy just to cut the section out and weld in a new piece of steel. We get our steel from the wrecked panels piled up behind J D’s Paint and Body Shop, but steel sheet metal is available at most metal or body shop supply stores. A trick we use, especially if the rusted area requires cutting around a body line, is to match up the same shape of the panel with one of the used panels, so we don’t have to shape it so much by hand.

4/13 <strong>2.</strong> The first step to properly repair body rust is to completely remove as much of the rusted area as possible, which means all of it. Any rust left in place can spread like cancer, corroding the adjacent metal and causing bubbles in the paintjob.

Of course another easy way to fabricate your metal repair patch is to purchase a new panel or just a patch-panel, and then simply cut the piece you need from the new panel. This method saves some time because your body contours and lines should line up properly, but it adds the cost of the new panel to the repair bill. Fortunately, body panels are relatively inexpensive for most of these cars, so we don’t mind spending the money to have the pre-stamped panel we can cut our repair patches from.

5/13<strong>3.</strong> While areas with severe rust or rust holes must be cut out completely, panels with surface rust can be ground down with a grinding stone or sand paper, and then chemically treated with a product like Ospho containing phosphoric acid. Phosphoric acid neutralizes iron oxide (rust), preventing it from re-forming after the car is painted.

Whether you decide to purchase the panel or fabricate your patch from sheet steel, it will need to be welded into the hole. We tend to prefer a wire feed mig-welder for this operation, set on a low voltage and wire-speed to create less heat. We then tack the patch in place and stitch-weld the panel in varying areas around the perimeter so the heat generated by the welding process doesn’t warp our patch or the adjacent metal. There are tools that will crimp the metal adjacent to the patch in order to form a recess, but making a nice tight fitting patch and butt-welding it in place is the preferred method. If priming the bare metal, be sure to use a weld-through primer in areas that will be welded.

6/13 <strong>4.</strong> Once the rusted area is cut away, like the lower quarter panel of this Coronet (you can now see the trunk drop extensions), there’s a good chance you’ll find more rust on the underlying structure. Here, we had to fabricate a patch for the trunk extensions so we’d have something to weld our outer patch to.

Once the metal patch is welded in place, the welds can be ground flush and the panel can be massaged with a hammer and dolly, but the repair will likely need a skim coat of body filler before being primed and painted. We’ve all heard claims that “this or that car is all metal, no filler”. And while these claims may or may not be true, we’ve found most repairs, no matter how skilful the body man or how accurate the patch, require at least a little fill to ensure straightness of the paintjob.

Speaking of paint, the best time to repair the rust in your Mopar is before you paint it, not afterwards. But if a small rust spot or hole does appear after your car is painted, don’t panic. Chances are you, a friend, or a competent body shop can repair the affected area and paint it to match without having to repaint the entire vehicle. Over time our cars will rust, but it’s nice to know that with the right parts, products, and procedures our Mopars can stay looking their best indefinitely.

7/13<strong>5.</strong> With the rusted area cut away, a metal patch will need to be fabricated to fit the area. New sheet metal is available from most body shop supply stores, or you can do like we do and cut the pieces you need from wrecked panels at the local body shop.